A school principal wants to observe how classroom area has an effect on classroom learning. The principal randomly chooses a group of students of the same age and grade, and places them in a classroom with an area of 400 square feet. The test scores of the class are recorded and analyzed at the end of the semester. How could this observation be changed into a valid experiment of the effects of classroom area? A. by including an identical classroom with a smaller number of students B. by including an identical classroom with students in a different grade C. by including a classroom with movable walls D. by including a classroom of identical area but a different shape